1. Field of the Invention
This invention relates to a video tape recorder capable of selecting a desired recording frame rate when an information signal is recorded. More specifically, the present invention relates to a video tape recorder that can simply and efficiently determine a recording mode by simultaneously recording both recording frame rate information and different types of time code information. In particular, the present invention provides means to accurately locate a reference point indexed by either a real time code or by a frame number while operating in playback mode.
2. Description of the Related Art
Existing video tape recorders (VTRs) used for digitally recording and reproducing both video and audio signals on magnetic tape, can also digitally record time information on the magnetic tape. The time information is useful as an index for playback and editing of the recording and it is typically coded using standard hours, minutes and seconds notation. For a more detailed description of the usefulness of time code based indexes or addresses for information recorded on tape, see U.S. Pat. No. 4,360,843 to Menezes et al. which is incorporated herein by reference. For example, a real time based indexing of a video tape would allow easy access to the precise frames that correspond to the start and end times of a broadcast program.
Thus, it is desirable to provide each frame with a unique time code for addressing purposes. In addition, it is also desirable to have time codes that accurately correspond to real time. Therefore, a time code would ideally be recorded for each frame. However, not all standard video signal formats have a frame rate that allows an integer number of frames to be recorded per each second of time.
For example, the National Television System Commission (NTSC) standard video signal format has a field frequency of 59.94 Hz. This means that the NTSC signal has a frame rate of 29.97 frames per second. In contrast, a high definition television signal used in a multiple sub-Nyquist sampling encoding (MUSE) system has a field frequency of 60 Hz. Thus, the high definition television signal frame rate is precisely 30 frames per second.
The consequence of not having an integer number of frames per second is that some frames cannot be assigned an hour, minute, second time code that corresponds to real time. Thus, in order to correct for the fraction of a frame that would otherwise occupy a different time code address, when a NTSC signal is recorded, two frames are skipped at the beginning of each minute except for every tenth minute. This difference adjustment allows each recorded time code to coincide with real time.
This frame-by-frame setting of the time code including the skipped frames is usually referred to as a Drop Frame (DF) stepping time code. The frames skipped to insure that the time codes correspond to real time are called “dropped frames”. The normal frame-by-frame setting of time codes, which is used for signal formats that have a frame rate that allows an integer number of frames to be recorded per each second of time, does not require a frame drop adjustment. Only whole frames are assigned a given time code. As mentioned above, the high definition television signal format has a frame rate of 30 frames per second and thus, does not require a frame drop adjustment to cause the assigned time codes to correspond to real time. This frame-by-frame setting of the time code in which it is not necessary to skip frames is usually referred to as a Non-Drop Frame (NDF) non-stepping time code.
In other words, because the time required to record or reproduce one frame of, for example, a high definition television signal which has 1125 scanning lines of resolution and a field frequency of 60 Hz is 1/30 of a second (or 33.33333 ms), the real time actually required when recording or reproducing exactly coincides with the non-stepping time code without having to execute any special adjustment operations.
In contrast, when an NTSC signal which has 525 scanning lines of resolution and a field frequency of 59.94 Hz is recorded or reproduced, the time required to record and reproduce one frame is 1/29.97 seconds (or 33.36667 ms). Therefore, the real time required to record, for example, 30 frames is longer than the one second increments of the time code. The result is that the time code lags behind the real time by 0.033367 ms for every 30 seconds of recording or play back time. Over an hour recording period, this lag adds up to a four second difference.
Thus, as indicated above, when a NTSC signal and time codes are to be recorded, a time code is recorded for each frame except that two frames are skipped at the start of each minute that is not a multiple of ten. In other words, two frames are skipped at the beginning of the first through ninth minutes, the eleventh through nineteenth minutes, the twenty-first through the twenty-ninth minutes, etc. As indicated above, drop frame stepping time codes are used so that the time codes coincide with real time during recording and reproduction.
Recently developed VTRs include the capability of selecting a correct frame rate based on a recorded signal. In other words, existing VTRs are capable of determining whether the field frequency of a recorded signal is either 59.94 Hz or 60 Hz. Likewise, existing VTRs can also determine whether stepping time codes (DF) or non-stepping time codes (NDF) have been recorded on the magnetic tape. Such VTRs can simultaneously determine the frame rate and the type of time coding.
As a result of there being at least two different frame rates and at least two different time coding methods, there are at least four different possible combinations of ways a recorded signal might be found on a magnetic tape. The relationship between the time codes of the individual combinations discussed above and real time are shown in FIG. 5.
As indicated above, in order to correctly select the proper playback mode in terms of frame rate and time coding, it is necessary to determine which combination of frame rate and time coding was originally used to record the signal on magnetic tape.
Referring to FIG. 5, when the recording frame rate is 59.94 Hz and the time code is recorded in an NDF format as specified on row (1) in the recording mode columns, one hour of real elapsed time results in a time coding count of 59 minutes, 56 seconds, and 12 frames. This count lags the real time in that the difference is three seconds and 18 frames. Therefore, when the playback frame rate is set to 59.94 Hz in the playback mode as indicated in row (1) of FIG. 5, a difference between the real time and the time code occurs. Hence, the recording time code cannot be used as a reference when it is desired to use a broadcast time, for example, as a control time.
On the other hand, when the information signal is recorded using a field frequency of 59.94 Hz and the stepping time code DF is used as indicated on line (2) in the recording mode columns of FIG. 5, one hour of real elapsed time results in a time coding count of one hour. This count completely coincides with real time. However, the number of frames to be reproduced is not an integer number (107,892.11 frames). This is a problem in that image editors, such as those used for animation and computer graphics, can only perform editing on whole frames. In other words, a frame is the smallest unit with which an image editor can work. When there are such fractions of frames involved, they are truncated and the result is that image transitions are not smooth.
Such a problem even arises when a high definition television signal is converted down into a ground wave and it is broadcast as a ground wave. In a VTR for HD which is capable of recording a high definition television signal (1125 scanning lines and 60 Hz field frequency) of the aforementioned MUSE system, the field frequency can be selected for recording as either 60 Hz or 59.94 Hz to preserve compatibility with a system that uses NTSC signaling (525 scanning lines and 60 Hz field frequency).
Referring to row (3) of FIG. 5, since a high definition television signal with a field frequency of 60 Hz is represented in 60 fields (or 30 frames) per second, the real time required for recording and reproducing coincides with the time displayed on the VTR's display counter if the step-by-step setting of time codes is NDF where frames are not dropped and the time codes are recorded in 1/30-second (=33. 33333 ms) units every frame.
On the other hand, referring to row (4), when a NTSC signal having a field frequency of 59.94 Hz is represented in 59.94 fields (or 29.97 frames) per second, the time code is delayed 3 seconds and 18 frames with respect to real time if the time codes are recorded in the 1/30-second units for each frame as mentioned above.
Thus, if no information about how to select the frame rate and the time coded is provided upon reproduction, then a process corresponding to real time and a process to be executed based on the number of frames as a reference, cannot be performed.
Further, when the recording frame rate is set to 59.94 Hz and the time code is recorded in the NDF format, the real time does not coincide with the time code as described in row (1) of FIG. 5, and the number of frames does not result in integer number of frames. If, for example, the playback field frequency is 59.94 Hz, which is identical to the rate used at recording, but a DF-type time code is used in place of the NDF type time code, then at least the real time coincides with the time code as described in line (2) of the playback mode columns of FIG. 5.
If the setting of the time code remains unchanged and only the playback field frequency is set to 60 Hz, then the time code coincides with real time and the number of frames becomes an integer as described in line (1) of the playback mode columns of FIG. 5. It therefore becomes possible to perform image editing work with smooth transitions.
In order to allow reproduction in a format different from that used for recording it would be desirable to have a VTR in which the playback frame rate can be arbitrarily selected and either NDF or DF time code formats can be also selected for playback information.